The present invention relates generally to the field of radiofrequency (RF) communications. More specifically, the present invention relates to integrated circuit (IC) power amplifiers.
A continuing need exists for lower power, less expensive, and physically smaller wireless devices to meet existing and future demands of portable and other electronic communications. Such demands are particularly prominent in cellular telephone, paging, wireless modem, and other applications. One area of inefficiency has been the power amplifier, which provides a signal amplification stage that operates on a radiofrequency (RF) signal prior to broadcasting the RF signal from an antenna.
Usually, benefits accrue in power amplifiers, as with other electronic devices, from forming as many electrical components together in a single integrated circuit (IC) as possible. When more components are included on a single IC, a physically smaller electrical device, reduced assembly times, reduced tuning requirements, reduced inventory management requirements, improved circuit reliability, improved circuit operation due to the use of matched components, and lower operating power may all be expected. Further benefits accrue when a given set of electronic functions can be carried out on an IC using less semiconductor die area. For example, a greater number of components can be produced from each wafer, thereby improving yields and reducing per-chip costs. Alternatively, a given die area can then accommodate more on-chip electronic functions.
A large number of communication applications is being developed to use RF signals in the 800 GHz-8 GHz range. The RF power amplifiers used in these applications would benefit if a greater number of electronic functions performed in and for RF power amplifiers could be performed on-chip and particularly if performed on-chip using as little die area as possible.
In the 800 GHz-8 GHz RF range, power amplifiers often require the use of multiple active devices, such as transistors. The formation of RF power transistors typically requires relatively little semiconductor die area. However, these power transistors are typically surrounded by input and output matching networks. Input matching networks match impedances between an incoming transmission line that brings the RF signal and the inputs of active devices, and the output matching networks match impedances between the outputs of the active devices and an outgoing transmission line which conveys the amplified RF signal to an antenna. Moreover, RF choking is typically used to provide biasing to the active devices and to decouple the active devices from one another. RF impedance matching networks and RF chokes typically use inductors, which are difficult to efficiently integrate with active devices in an IC.
Inductors have been difficult to integrate because a usable amount of inductance for the 800 GHz-8 GHz RF range has conventionally required an undesirably large amount of die area. Consequently, conventional RF power amplifiers typically rely upon some off-chip inductive components. For example, a couple of matching networks may be formed on-chip when each network requires only a single inductor exhibiting an inductance of no more than few nanohenries. However, power amplifier designs which use several inductors, some of which may need to exhibit greater inductance values typically rely on one or more off-chip inductors. For example, each RF choke which provides biasing and decoupling for an active device typically exhibits an inductance value of more than 20 nanohenries and is located off-chip. Not only does the high inductance value make an off-chip implementation the more efficient option, but other off-chip tuning circuits may then be employed to minimize the influence the decoupling/choke inductor may have on impedance matching circuits, a complex influence often difficult to quantify prior to manufacture.
Transformers represent a class of inductive devices that may be used to match impedances. However, conventional on-chip transformer-forming techniques lead to either the consumption of an undesirably large amount of die area or an insufficient amount of inductance to achieve good magnetic coupling without suffering significant losses. Accordingly, using conventional techniques on-chip transformers provide little benefit over matching networks in the 800 GHz to 8 GHz RF range.
It is an advantage of the present invention that an improved monolithic balanced RF power amplifier is provided.
Another advantage of the present invention is that an integrated RF power amplifier is provided which uses an on-chip transformer both as a balun for input and/or output and to provide biasing to active devices.
Another advantage of the present invention is that an integrated RF power amplifier is provided which uses on-chip transformers as input and output baluns and to provide biasing to active devices.
Another advantage of the present invention is that an integrated RF power amplifier is provided which uses an on-chip transformer configured in an efficient manner so that losses are reduced and coupling between primary and secondary windings is increased.
These and other advantages are realized in one form by an improved monolithic balanced radiofrequency power amplifier. The power amplifier includes a semiconductor substrate. First and second transistors and a transformer are formed over the substrate. The transformer has a first conductive spiral coupled to the first transistor and residing over a first portion of the substrate so that positive current flows in a first rotational direction. A second conductive spiral couples to the second transistor and resides over a second portion of the substrate so that positive current flows in a second rotational direction which opposes the first rotational direction. A center tap resides between the first and second conductive spirals. A third conductive spiral is positioned over the first portion of the substrate, and a fourth conductive spiral is positioned over the second portion of the substrate. A source of direct current biasing for the first and second transistors couples to the center tap of the transformer.